Written evidence submitted by Professor Alastair Lewis (HEN0003)

National Centre for Atmospheric Science (NCAS)[1] and University of York

 

Declaration of interests.

Prof Alastair Lewis is an employee of the University of York and is currently:

Chair of the Defra Air Quality Expert Group (AQEG)

Chair of the Department for Transport, Science Advisory Council

Chair of the NERC Science and Innovation Advisory Committee

Member of the Environmental Sustainability Panel of the Civil Aviation Authority.

 

Evidence related to adequacy of the HENPS in relation to air quality issues.

Assessment of impacts

  1. This submission reviews the adequacy and measures included in the HENPS related to air quality and potential mitigations. It draws on recent evidence and science advisory activities of the Air Quality Expert Group, an Expert Committee to Defra, but is submitted here in a personal capacity and is not intended to represent the views of that committee.

 

  1. AQEG is currently working on a report to Defra evaluating how airports in the UK play impact on local and regional air quality and will likely submit that report to Defra towards the end of 2026.

 

  1. Air pollution is multifaceted and encompasses a broad range of entities that are harmful to health when inhaled. The fine particulate matter metric PM2.5 and nitrogen dioxide (NO2) receive the most attention in terms of reporting, campaigning and policy debate, but many other pollutants are present that cause harm. These include carcinogenic volatiles such as benzene and formaldehyde, carbon monoxide, sulfur dioxide, trace metals and persistent organic pollutants (POPs). The impact of airports air quality has been summarised in this POST document from earlier in 2026[2].

 

  1. Ultrafine particles (UFP) are a particularly important air pollutant from airports since they are emitted in large amounts in the exhaust of aircraft engines. They are a subset of smaller particles (smaller than 0.1 micrometres diameter) contained within the PM2.5 metric (all particles smaller than 2.5 micrometres diameter). Whilst they are comprise only a small amount of particle mass, they are considered by the World Health Organisation to be of toxicological significance and can penetrate deep into human lungs. The recently updated European Union Air Quality Directive (2024/2881) now require UFP to be monitored in higher risk locations, although no annual limit value has been set.

 

  1. The HENPS correctly identifies that the most significant direct local air quality effects of airports are the emissions from aircraft engines and auxiliary power units (APUs) that occur during aircraft idling, and during the landing and take-off cycle (which includes taxiing to take-off). There are further linked air quality impacts from ground handling vehicles, airport buildings infrastructure (e.g. space heating) and the connecting of people, freight and airports to the wider transport system.

 

  1. Section 5.6 of the HENPS sets out the wider context of emissions and concentrations of NOx and PM. At present UK NOx emissions and concentrations are falling due to road transport fleet modernisation and electrification and decarbonisation of power generation. Adoption of heat pumps is expected to further reduce NOx emissions. This is a very positive trend. However, the negative health impacts of NO2 are now viewed as occurring at lower concentrations and exposures than previously thought, with WHO recommended annual mean guidance for exposure falling from 40 mg m-3 to 10 mg m-3 in their most recent air quality and health evidence assessment in 2021. EU air quality standards have now reduced annual limits on NO2 from 40 to 20 mg m-3. The UK continues to maintain an annual mean limit value of 40 mg m-3, which is largely now being met in most UK locations, including around Heathrow. 

 

  1. The HENPS uses two broad benchmarks for assessment of acceptability of air quality impacts arising from airport expansion. These reference i) the National Emissions Ceiling Regulations (NECR) (2018) which limit total amount of emissions at the national level of NOx and PM2.5, and ii) standards for ambient concentrations contained within the Air Quality Standards Regulations (2010) with further limits on PM2.5 set out in the Environment Act 2021, and The Environmental Targets (Fine Particulate Matter) (England) Regulations 2023.

 

  1. The HENPS guidance for assessment applies a legal interpretation of minimum statutory air quality requirements as set out in existing law. An inadequacy of this approach for the purposes of planning is that it treats air pollution harms as being subject to either a pass or fail test, which does not represent the scientific evidence of harms to the population. Indeed it does not represent the damage costs of incremental additions of air pollution, as set out in the Treasury Green Book, and it is unclear that these costs would be taken into account in any impact appraisal.

 

  1. Sections 5.1 to 5.10 (and later 5.25) of HENPS create a permissive environment that would allow for airport expansion to have licence to pollute ‘up to the limit’ of either NECR or annual mean limit values for ambient concentrations of NO2 or PM2.5. The HENPS in sections 5.1 to 5.10 does not impose a duty on developers to maintain or improve on existing air quality around the airport, although this is potentially contradicted in 5.17 under mitigations. Since aircraft mostly emit very small particles, as UFP, and rather few larger ones, more aircraft movements are unlikely to impact substantially on PM2.5 as a metric, but will they impact on NOx.

 

  1. No back sliding on ambient air quality is an important environment and public health principle since the overwhelming evidence on the health effects of air pollution indicates that harms are on a continuum and that for starting concentrations typical of around Heathrow (and central London), any increase in air pollution will undoubtedly lead to increased health harms to local populations. This may be legally permitted, but it unequivocally creates quantifiable economic and social costs.

 

  1. By taking an approach that allows additional pollution ‘up to the limit’ there is potential for Heathrow expansion to offset recent improvements in air quality that have occurred due to investment in clean technology in other sectors, and including investment made directly by the public. NO2 concentrations in the UK are falling in large part due to investment in electric vehicles, however since HENPS does not stipulate explicitly no regression in ambient NO2 air quality, those air quality gains made (and paid for) by others can be undone by the airport choosing to expand. By 2040 it is estimated that more NOx will be released from UK airports than from the entire passenger car fleet of the UK.

 

  1. Of particular concern is that operator adoption of various mitigation measures (beginning section 5.11, and also para 5.25) could be interpreted as only necessary should a pre-existing legal standard be exceeded or be likely to be exceeded. It would, as written, in 5.1 to 5.10, permit do-nothing until limit values for NO2 or PM2.5 were close to being breached.

 

  1. Since HENPS frames air quality impacts as significant only in the context of pre-existing statutory requirements, it largely discounts in the assessment guidance the evidence of harms related to increased UFP emissions, for which there is no current air quality standard for England. The evidence is very clear that health harms from UFP occur whether or not a legal air quality standard exists in England. With knowledge of the scientific evidence of harms from UFP and that airports and aircraft are a major source of UFP, then more explicit limits on both emissions and concentrations should be included within the HENPS iassessment. It is welcome that UFP monitoring is included within the package of mitigation measures (5.23), but with no specific targets it is unclear what responsive actions the monitoring data would stimulate.

 

  1. Setting explicit standards or requirements for UFP is important not solely for residents living close to Heathrow. Whilst NOx emissions from the airport have greatest impacts on air quality in the few kilometres from the airport boundary, the atmospheric lifetime of UFP is longer and they can impact on air quality downwind, including on residents living in central London under certain atmospheric conditions. 

Mitigation measures

  1. The HENPS contains a comprehensive range of reasonable mitigation measures, and it is welcome that air quality impacts during construction of any additional capacity or runway are explicitly considered within the plan. These could be significant and long-lasting, and the mandated use of low tailpipe emission, or zero emission construction equipment would be impactful both for air quality and greenhouse gas emissions. Measures to manage and reduce dust and windblow soil particles will also be essential.

 

  1. Section 5.17 is somewhat at odds with other parts of HENPS related to air quality. The 5.17 requirement to introduce mitigations where a deterioration in air quality is anticipated would be a more robust protection of public health and arguably meet the intentions of the Environment Act (2021) and Population Exposure Reduce Target (PERT). The PERT creates an obligation that there is no overall (nationally averaged) deterioration in PM2.5 air quality, ensuring that those locations currently below the annual mean limit value have their existing air quality protected and improved long-term. Arguably, the principles and intentions of the Act, and the PERT specifically, should apply locally to Heathrow, and that mitigations should be put in place to ensure no backwards steps from the current position.

 

  1. This submission will not review all of the mitigation measures for their effectiveness, but three aspects will be highlighted in paras 18-20 as being particularly significant.

 

  1. The greenhouse gas and noise impacts of increased Heathrow capacity are determined largely by the change in overall volume of flights departing and arriving. However, a substantial fraction of the air quality impact derives from the operational use of aircraft engines and APUs at low power settings on the ground and this does not necessarily scale linearly with number of aircraft movements.

 

  1. Actions to reduce the use of APUs during time spent on stand and reduced overall taxi times are likely to have significant impacts on total emissions of NOx and UFP, and their ambient concentrations around the airport. It is essential that the effects on projected taxi time after additional runway capacity is added are fully modelled for air quality. Plausibly, an airport that had more flights, but with very efficient (potentially electric) taxiing and with limited queuing delays, might actually have lower local air pollution emissions than the present day. 

 

  1. Landing charges that reflect air pollution emissions are already used at some UK airports (including Heathrow and Gatwick), and a toughening of the stringency of these to incentivise the use of the most modern and cleanest aircraft would clearly be beneficial for local air quality.

Note on future use of sustainable aviation fuel (SAF).

  1. The current Department for Transport strategy for aviation decarbonisation and reduction in climate impacts leans heavily on the use of SAF as a means to reduce the fossil carbon intensity of flying, enacted via the SAF Mandate (2024). This may possibly be supplemented in the future by measures associated with non-CO2 impacts such as contrail avoidance, although the scientific uncertainties associated with the effectiveness of such interventions are large. 

 

  1. Using SAF as a fuel may reduce net UK greenhouse gas emissions, but air pollution is still generated from the aircraft engine. SAF is not a solution to the air quality impacts of aviation. In January 2026 the Defra Air Quality Expert Group provided a short advice note to Defra on the possible impacts of SAF adoption on emissions of key air quality pollutants. A section of that letter is reproduced below for information.

 

Nitrogen oxides (NOx). Aircraft engines are a major source of NOx emissions, and these are hard to abate since they form as an inherent consequence of high temperature combustion. It is not possible to add exhaust aftertreatment to aircraft engines. Changing the formulation of fuel from kerosene to SAF would be expected to have only a small, if any, impact on NOx emissions from any given engine. Nonetheless, more modern high air by-pass ratio engines do emit significantly less NOx than older engines, and so natural aircraft fleet turnover will have a positive impact in reducing NOx emissions, irrespective of fuel used. Absolute NOx mass emissions are often high when engines operate under high loads, for example during take-off. Under these conditions large amounts of NOx are released but diluted in very large volumes of air passing through the engine. Concentrations on NOx in exhaust gases can be high during taxi when there is less air flow as diluent through the engine, so ground delays can impact on local air quality. In a future where SAF is the majority fuel, substantial ground level NOx emissions are likely to remain.

 

Fine particulate matter. Aircraft engines using Jet A-1 release relatively small amounts of particles as measured by mass, or the PM2.5 metric, but they are a major source of ultrafine particles (UFP). UFP are an important class of particulate pollution with a growing body of evidence identifying health impacts. The emissions of UFP from aircraft engines have been shown to spread well beyond airport boundaries and can lead to elevated UFP in nearby cities. Some evidence indicates that a switch from Jet A-1 to SAF may possibly lead to lower UFP emissions, however it is very likely that SAF fuelled aircraft will always remain a source of some UFP.

 

Black carbon (soot). A subset of particulate matter emitted from aircraft is black carbon, which has both direct health and climate impacts in its own right. The formation of black carbon is thought to be influenced by the aromatic content of fuels, found in the form of monoaromatic and naphthetic hydrogens. Higher aromatic content can lead to higher BC emissions. SAF is typically composed of mostly straight chain hydrocarbons rather than aromatics and is thus less susceptible to BC formation. It is now well-established that there are lower BC emissions from SAF fuelled engines. However aromatic compounds are sometimes needed in aviation fuel to maintain the integrity of elastomer seals. It is possible that aromatics may be deliberately added back in to SAF to support backward engine compatibility. Should that occur, the notional benefit of lower BC from SAF would be reduced.

 

Sulfur dioxide. Current Jet A-1 fuel contains some sulfur (up to 3,000 ppm is allowed), originating from the fossil fuel source and left over from the refining process. This sulfur is combusted and is a source of SO2 from aircraft engines, including at ground level during idle and taxi. SAF generated from biogenic feedstocks (e.g. from vegetable oils, wood digestion etc) does not naturally contain any sulfur and so a SAF-fuelled aircraft engine would be expected to have essentially zero SO2 emissions - an air quality benefit. However, some potential SAF feedstocks, such as non-recyclable municipal wastes may contain sulfur and this may propagate through to the finished fuel. There are however high levels of uncertainty in this, but it is a factor to monitor as different SAF production methods come on stream.

 

In addition to forming SO2, fuel sulfur contributes a key precursor to ultrafine nanoparticles emitted by engines. Between ~ 1 – 3% of the sulfur is oxidised and nucleates as sulphate particles, initially of <10nm in size. Emerging research suggests that the volatile fraction of particles from the sulfate can contribute to up to 50% of the total UFP concentration at low thrusts. Additionally, modelling studies suggest that reducing BC as a consequence of a low/zero aromatic fuel may not reduce the climate impacts of aircraft as contrails will form on the volatile fraction in the absence of BC.

 

Non-methane volatile organic compounds (NMVOCs). NMVOCs are emitted from aircraft engines particularly at low loads during idle and taxi, a consequence of incomplete combustion. Emissions can include unburned fuel and partially combusted oxygenated VOCs. The impacts of SAF on NMVOC emissions are complex. A lower aromatic content in SAF may be beneficial for air quality since these compounds have high ozone and PM forming potentials. On the other hand, the higher linear aliphatic and oxygenated hydrocarbon content in SAF may lead to more efficient formation of small, oxygenated by-products such as formaldehyde and acetaldehyde which have direct air quality impacts. This is an area with considerable uncertainty, and where a mix of positive and negative impacts might emerge.

 

Ammonia. Aircraft engines are not currently considered to be a source of ammonia, and this appears unlikely to change through the adoption of SAF in place of Jet A-1.

 

July 2026

 

Endnotes

[1] The National Centre for Atmospheric Science is one of the Natural Environment Research Council's (NERC) established research centres. The Centre increases knowledge of key environmental issues including climate change, weather processes and atmospheric composition including air quality.

[2] https://post.parliament.uk/the-health-impacts-of-airports-on-local-residents/